1. Field of the Invention
The present invention relates to a voltage-controlled oscillator of which the oscillation frequency is varied according to a tuning voltage, and in particular to a voltage-controlled oscillator for use in a high-frequency communication apparatus or the like.
2. Description of the Prior Art
Voltage-controlled oscillators are generally used in high-frequency communication apparatus and the like, particularly as oscillators for frequency synthesizers serving as local oscillators in wireless communication apparatus. Now, a conventional voltage-controlled oscillator will be described.
FIG. 4 shows a typical configuration of a conventional voltage-controlled oscillator. The voltage-controlled oscillator includes a resonant portion 1 and an active portion 2.
The resonant portion 1 is a circuit having an inductor L1 and a variable-capacitance device C1, such as a variable-capacitance diode, connected in parallel. The symbol RT represents the sum of the parasitic resistance of the inductor L1 and the parasitic resistance of the variable-capacitance device C1. One of the nodes at which the inductor L1 and the variable-capacitance device C1 are connected together is connected to an output terminal S1, and the other is connected to an output terminal S2.
A reference voltage supply terminal 4 is connected to the half point of the inductor L1. A reference voltage VCC is supplied to the reference voltage supply terminal 4. The capacitance of the variable-capacitance device C1 is controlled by a tuning voltage Vtun that is applied to a control terminal 5.
The active portion 2 includes NPN-type transistors Q1 and Q2, bias resistors R1 and R2, signal level adjustment capacitors C21, C22, C31, and C32, and a DC (direct-current) current source 3.
The collector of the transistor Q1 is connected to the output terminal S1, and the collector of the transistor Q2 is connected to the output terminal S2.
The base of the transistor Q1 and the collector of the transistor Q2 are connected together through the capacitor C21, and the base of the transistor Q2 and the collector of the transistor Q1 are connected together through the capacitor C22.
The base of the transistor Q1 is connected through the bias resistor R1 to a bias voltage supply terminal 6, and is also grounded through the capacitor C31. The base of the transistor Q2 is connected through the bias resistor R2 to the bias voltage supply terminal 6, and is also grounded through the capacitor C32.
The emitters of the transistors Q1 and Q2 are connected together, and are grounded through the DC current source 3.
The voltage-controlled oscillator configured as described above operates as follows. The resonant portion 1 outputs a signal having a given resonant frequency to the output terminals. The negative resistance of the active portion 2 is canceled by the resistance RT and the load resistance connected between the output terminals S1 and S2, and thus the output voltage Vo appearing between the output terminals S1 and S2 is a standing wave. In this way, it is possible to obtain an output voltage Vo having the desired frequency according to the Vtun.
One of the parameters that describe the performance of the resonant portion 1 is the Q factor. If the impedance of the resonant portion 1 is assumed to be Z, then the Q factor is given by formula (1) below.
Q=Image (Z)/Real (Z)xe2x80x83xe2x80x83(1) 
On the other hand, if the offset frequency from the oscillation frequency is assumed to be xcex94f, then the phase noise n(xcex94f) of the voltage-controlled oscillator is approximated by formula (2) below.
n(xcex94f)xe2x88x9d1/[Q2xc3x97(xcex94f)2]xe2x80x83xe2x80x83(2) 
Hence, the higher the Q factor of the resonant portion 1, the more the phase noise can be reduced. For this reason, it is generally believed that portable communication apparatus require the use of voltage-controlled oscillators of which the resonant portion has a high Q factor.
On the other hand, in portable communication apparatus, to make them compact and inexpensive, voltage-controlled oscillators need to be formed as integrated circuits. However, monolithic passive devices have low Q factors, and therefore forming the resonant portion of a voltage-controlled oscillator as an integrated circuit lowers the Q factor of the resonant portion, degrading the phase noise characteristics of the voltage-controlled oscillator.
Japanese Patent Application Laid-Open No. H10-145190 proposes to solve this problem by adopting a circuit configuration that reduces phase noise, and by forming an inductor out of bonding wire.
FIG. 5 shows a voltage-controlled oscillator disclosed in Japanese Patent Application Laid-Open No. H10-145190 mentioned above. Here, such circuit elements and signals as are found also in FIG. 4 are identified with the same reference numerals and symbols, and overlapping explanations will not be repeated.
The voltage-controlled oscillator of FIG. 5 differs from the voltage-controlled oscillator of FIG. 4 in the following respects. The inductor L1 is replaced with inductors L11 and L12. The variable-capacitance device C1 is replaced with variable-capacitance diodes C11 and C12. The DC current source 3 is replaced with a resistor R3. The capacitors C31 and C32 are abolished. Between the inductor L11 and the variable-capacitance diode C11, a high-pass filter 7 composed of a capacitor and a resistor is inserted. Between the inductor L12 and the variable-capacitance diode C12, a high-pass filter 8 composed of a capacitor and a resistor is inserted. The cathodes of the variable-capacitance diodes C11 and C12 are connected together, and are then connected through a resistor R4 to the tuning voltage supply terminal 5.
In the voltage-controlled oscillator of FIG. 5, the high-pass filters 7 and 8 provided in the resonant portion serve to separate the resonant portion from low-frequency fluctuation of the reference voltage VCC, thereby reducing phase noise. However, the provision of the high-pass filters 7 and 8 in the resonant portion tends to lower the Q factor of the resonant portion as a whole and thus eventually increase phase noise n(xcex94f).
Japanese Patent Application Laid-Open No. H10-145190 mentioned above also discloses a configuration in which, for the sake of integration, the inductors L11 and L12 are replaced with bonding wires. This configuration, however, suffers from lower reliability of the oscillator, when mounted, and increased variations in the inductances of the inductors.
An object of the present invention is to provide a voltage-controlled oscillator that offers satisfactory phase noise characteristics even when formed as an integrated circuit.
To achieve the above object, according to the present invention, a voltage-controlled oscillator is provided with a resonant portion including at least one variable-capacitance device and at least one inductor, an active portion including differential pair transistors and supplying electric power to the resonant portion in such a way as not to attenuate the oscillation of the resonant portion, and a current source connected to the emitters or sources of both of the differential pair transistors. Here, the current source is a circuit having a resistor and a capacitive device connected in parallel.